Stamped by machine on a writable compact disk is a wobbled groove. The groove provides means for tracking on the disk while writing or reading data that is written in the groove. The wobbled groove, which is a frequency modulated signal after detection and processing, contains addressing o and other information that are necessary for the write and read processes of a CD writer/reader.
Information is coded in a series of blocks, each of said blocks comprising 42 bits: four bits of a sync mark, eight bits of "minutes", eight bits of "seconds", eight bits of "frames", and fourteen bits of "CRC" (cyclic redundancy check). For a more complete discussion of this coded information and its extraction see commonly assigned U.S. Pat. No. 5,363,360, issued Nov. 8, 1994 to Fairchild. In any event to extract the information from the wobbled groove, the following process is necessary:
1. The laser beam associated with the optical head uses a tracking servo to attempt to follow the wobbled groove, but the servo is bandwidth limited to approximately 5 Khz. At a 6X CD data rate, the wobbled groove frequency is 132.3 Khz. Obviously, the tracking servo cannot follow the wobbled groove which creates an error voltage in the tracking error signal. The error signal is filtered and amplified to produce a TTL level, frequency modulated signal called ATIP FM data.
2. The ATIP FM data is input to a demodulating phase locked loop which extracts biphase encoded data (biphase data rate=18.9 Kbps). PA1 3. The biphase encoded data is input to another phase locked loop which recovers a 2X clock (2X clock frequency=37.8 Khz). PA1 4. The biphase encoded data and the 2X clock are input to an ATIP decoder which detects the sync mark, converts the biphase encoded data to NRZ (non-return to zero) data, and provides the status of the ATIP subsystem. PA1 (a) extracting an FM signal from the wobbled groove; PA1 (b) providing a high frequency phase-lock loop which responds to the extracted FM signal to produce a high frequency clock; PA1 (c) converting such FM signal into biphase data by: PA1 (d) extracting a clock signal from a digital phase-lock loop responsive to the biphase data; and PA1 (e) providing an ATIP decoder which in response to the biphase data, biphase clock, and high frequency clock signal provides sync detection, address information, and ATIP decoder status.
Unfortunately, the phase locked loops which are necessary for demodulation and clock recovery are constructed using analog design and components. The components require extensive printed circuit board space and add cost to a CD writer/reader. In addition, the demodulating phase locked loop adds considerable delay to the biphase encoded data relative to the physical location of the data on the disk.